Hi Phil,
To help you further with your goal keeper problem of moving too soon toward the front post, please see the new Goal Keeping videos - "FIELDING CROSSES - PART 1,2 and 3) that I have just added to our Video library.
The 1st video focusses on crosses or approaches by players into / toward the front post and will help your goal keeper understand how to position themselves correctly, the other 2 focus on out-swinging crosses into the 6yds box and in-swinging lofted pases into the back post.
Teach him WADCA - The videos and session plans will explain what that means, it will help you and him resolve the problem you have.
regards
Richard